The invention relates to filler for an elongated optical transmission element which contains at least one optical waveguide and at least one protective covering enclosing the optical waveguide, the filler having a component in the form of a plasticizer.
U.S. Pat. No. 4,342,500 discloses a cable which is high voltage-resistant and can therefore be used in the region of high voltage installations. If such a cable were filled only with crosslinked synthetic materials (polyurethane, epoxy resin, polyester resin, see column 1, lines 58 and 59), air gaps would then be unavoidable. Owing to the high field strength concentrations, these air gaps would lead to difficulties. For this reason, these existing plastics materials are plasticized by the addition of appropriate plasticizers, because it is possible in this way to avoid the formation of such undesirable cavities. Thus, the addition of plasticizers in this prior art reference has the sole purpose of plasticizing the existing plastics material sufficiently to achieve complete filling of all gaps with the plasticized plastics material and thus to overcome the difficulties with the high voltage fields.
WO 92/00368 discloses a filler for optical waveguide cables, which filler may contain polypropyleneoxyglycol (PPG), as is evident from the examples in Table 1 on page 16 and from Examples 6-11 disclosed on page 14. However, these polypropyleneoxygiycols may be considered to be suitable for use only when having molecular weights of at least 3000. Molecular weights between 3000 and 8000 are preferably used, as disclosed in claim 1 of the reference. Page 14, lines 22 to 24, discloses that embodiments 6 and 7 are not suitable for use and that molecular weights of more than 3000 are therefore used.
EP 029 198 91 discloses a filler for optical transmission elements which comprises a mixture of an oil and a thixotropic agent and additionally contains an organic thickener which completely or partially comprises halogenated and/or halogen-free hyarocarbon polymers. Other fillers likewise containing oils are described in DE 38 39 596 A1 and U.S. Pat. No. 4,701,016. German Offenlegungsschrift 27 28 642 discloses a longitudinally water-tight optical waveguide cable in which swollen polystyrene in oil is used as a lead filling material or wire filler. GB-A 2 059 097 discloses an optical cable which contains a cable sheet comprising stabilized PVC material. Such cable sheets must be flexible and in particular the cable sheet cannot become rigid when low temperatures are encountered. In order to ensure sufficient flexibility at temperatures down to -50.degree. C., the PVC compound of the cable sheet contains a di(2-ethylhexyl) sebacate (DOS) plasticizer. Specifically, the PVC compound may contain 45 to 70 parts by weight of the DOS plasticizer, 10 to 50 parts by weight of a filler and 3 to 8 parts by weight of stabilizers, relative to 100 parts by weight of the PVC. This compound is converted into granules, which can be applied to a cable core containing optical waveguides by means of an extruder. This prior art states nothing about the use of a filler, i.e., a material which can be arranged between a cable sheet and the optical waveguides.
The requirements which such wire fillers have to meet are that the optical waveguides are as far as possible not subjected to any impermissible tensile and/or compressive forces, i.e. the filler must not be too rigid. Since this should also be the case at relatively low temperatures, for example -30.degree., the oils used have to meet particularly high requirements so that they do not harden excessively at the low temperatures. Furthemore, these fillers should be sufficiently resistant to dripping out (drip test), and this is a critical requirement particularly at relatively high temperatures. By using thixotropic agents in the known wire fillers, it is possible to advantageously influence the behavior, particularly at relatively high temperatures, to which the use of thickeners finally also contributes. However, the disadvantage of the known fillers is that their molecular composition is relatively non-uniform, owing to the different base components used. Furthermore, expensive base oils must be used if the wire filler is to meet high requirements. Moreover, conventional fillers also interact with polyolefins, i.e. exhibit an increase in mass of &gt;5% by weight. As a result, the properties of the polyolefin wire sheaths change.